This happened to me two summers ago. It was horrifically hot and high humidity, and a friend of mine needed to get a fence completed before the end of the week. We did everything right: regular hydration, loose protective clothing, broad hat, not working too hard. None of it made any difference. After just two hours of pottering about I started feeling nauseous and weak, I couldn't even carry a shovel or walk in a straight line. Because we were in a remote location, my friend tried to convince me that we should quit for the day (he wanted to take me to the hospital). Instead, all I did was sit under a shady tree for about two hours, and when we had a thunderstorm in the afternoon the temperature cooled and I started improving. I was really weak for about two days, and spent a fair bit of that time in bed. I'm nearly fifty now, which may have attributed to my condition, but when I was younger I used to do this sort of activity for a living and never had a problem.